1. The Field of the Invention
The present invention relates to impedance synthesis. More specifically, the present invention relates to the synthesis of user specified source or load impedances using digital processing.
2. The Prior State of the Art
Ordinarily, circuits are designed such that the load impedance is much greater than the impedance of the source that is driving the load. Otherwise, the load impedance may have an adverse effect on the source voltage by causing the output voltage of the source to drop. This undesirable result is related to the finite value of the source impedance. Transmission lines, however, are an exception to this general rule. In the case of transmission lines, it is desirable that the load impedance match the impedance of the transmission line for several reasons.
In a basic form, a transmission line is two or more parallel conductors which connect a source to a load. The load presents an impedance to the transmission line and the transmission line presents a characteristic impedance, which is usually a combination of the source impedance and the impedance of the transmission line, to the load. When the transmission line is attached to a load having an impedance equal to the characteristic impedance of the transmission line, the power in the signal transferred to the load is maximized and the signal is not reflected back to the source. These benefits are important for many different applications. If the power transfer is not maximized, it is possible that the connecting device will be unable to properly interpret the signal. If signal reflections are present on the transmission line, then the signal becomes difficult to demodulate and additional circuitry is required to remove the reflections or echoes.
One common example of a transmission line which is used for moderate frequencies is a parallel conductor, which is frequently used in telephone networks. The parallel conductors of a telephone network are often referred to as the tip and ring. Thus, the tip and ring comprise the transmission line and the load impedance may be embodied as a telephone, modem or other device capable of connecting to the telephone network.
The telephone network specifies the characteristic impedance of the transmission line which must be matched by a connecting device in order to fully transfer power and avoid signal reflection. However, the impedance specified by the telephone network is usually only an approximation of the actual impedance, which results from such variables as: the variations in the length of the transmission lines to the connecting device from the central office; various wiring topologies within an intermediary installation such as a series of parallel transmission lines within a business or other structure; and intrinsic variations in the transmission lines themselves. The actual characteristic impedance presented by the telephone network is difficult to precisely match and is usually only approximated.
With regard to telephone networks, the problem is complicated by the fact that telephone networks across the world specify different characteristic impedances. In this situation, it is feasible that a device functioning perfectly in one telephone network will encounter difficulty in another telephone network. Because telephones, modems and other telephonic devices are being used world wide, it is necessary to enable a telephonic device to function in any telephone network environment. While many devices are capable of operating in different networks, the result is not always satisfactory. One solution is to characterize the impedances of the various telephone networks into groups and physically place more than one impedance in the device. The appropriate impedance is then selected using appropriate switching technologies such as relays or field effect transistor (FET) switches. This method has several disadvantages. First, control circuitry must be employed to control the relays and switches, which is not a trivial task because of the high voltages which may be present on many transmission lines. Because of the high voltages, the components used for the switches and relays can be large and expensive and must be rated to withstand the high voltages which can be present on a transmission line.
While placing multiple impedances on a device to permit a device to function in more network, the physical impedances physically placed on the device are designed to approximate, rather than match, the characteristic impedances that may be encountered in different telephone networks, which results in less than optimal power being transferred to the load as well as signal reflections back to the signal source. Also, many devices, such as modems, have limited printed circuit board surface area on which to place these additional circuit elements and a relatively large number of discrete circuit components such as resistors, operational amplifiers and capacitors can require significant surface area. Further, the combined tolerances of the passive and active circuit components may result in a large variance from the desired impedance.
The problem of properly terminating a transmission line has also been addressed in terms of impedance synthesis. However, these attempts have involved the use of discrete circuit components such as resistors and operational amplifiers. These methods, however, are limited to synthesizing real or resistive impedances. Recursive digital filters have also been utilized, but this approach introduces incidental shunting impedances, whose effects must be eliminated. In addition, digital filters are capable of introducing unacceptable delays.
In view of the foregoing and other problems in the prior art, it is therefore an object of one embodiment of the present invention to provide a system and method that can synthesize an impedance.
Another object of one embodiment of the present invention is to provide a system and method that accomplishes impedance synthesis by substantially matching a load impedance.
Yet another object of one embodiment of the present invention to synthesize a specified source impedance.
It is yet another object of one embodiment of the present invention to synthesize negative impedances.
It is a further object of one embodiment of the present invention to gyrate inductive and capacitive circuit elements.
It is another object of one embodiment of the present invention to synthesize prescribed impedance across a pair of terminals.
In summary, these and other objectives are obtained with embodiments of the present invention that provide systems and methods for synthesizing a prescribed impedance either across a pair of terminals or at a source. In general, generating or sinking a current synthesizes the impedance, such that the ratio of the voltage to the current is the prescribed impedance. The synthesis of impedance has particular application in at least two general instances: load impedance synthesis and source impedance synthesis. Impedance synthesis is not, however, limited to these examples, but can be adapted to many different circumstances. Because the signal is attenuated, it does not interfere with the operation of the network connected device. Embodiments also provide the ability to synthesize desired termination impedance, so that the network connected device matches the characteristic impedance of the network.
Load impedance synthesis usually occurs in the context of a transmission line. The transmission line exhibits an associated characteristic impedance that should be matched by the load in order for the power transfer to be maximized and in order to avoid signal reflection back to the source. To synthesize the known load impedance, the voltage across the transmission line terminals is measured. This measured voltage is then converted to its digital equivalent. A digital processor processes the digital equivalent according to a scaling factor related to the prescribed impedance. The output of the digital processor controls a current source and causes the current source to generate a current having a value of the measured voltage divided by the prescribed impedance. This causes the prescribed impedance to be synthesized across the terminals because the measured voltage divided by the generated current is equal to the prescribed impedance.
Source impedance synthesis operates in a similar manner. The difference is that the voltage is digitized in this case is the source voltage minus the load voltage. In both cases, the current source or voltage to current converter may be bi-directional, in which case negative impedances are easily synthesized. Also, because a digital processor is utilized to properly scale and process the measured voltages, inductive elements can be easily gyrated to capacitive elements and vice versa.
The present invention gains a significant advantage because of the ability to incorporate a digital processor, memory, analog to digital converters, and digital to analog converters into a single application specific integrated circuit (ASIC), which are already present in many devices. Surface area on devices such as PCMCIA compliant modems is precious and the ASIC ensures that the only component that need be added to such a device is a current source. In some instances, the current source can be internal to the ASIC, which eliminates the need for external components as long as the maximum voltage of the transmission line does not exceed the safe operating voltage of the ASIC. Also, impedances synthesized by the present invention are much more accurate, assuming that the arithmetic of the ASIC has high resolution, than conventional circuit elements, thus avoiding the unknown effect of the tolerances present in physical circuit elements.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by the practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instruments and combinations particularly pointed out in the appended claims. These and other objects and features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.